Field of the Disclosure
The field of the disclosure is that of signal transmission systems and methods.
More specifically, the disclosure relates to methods and systems for transmitting signals on a multi-conductor cable comprising a plurality of conductors surrounded by an armor, the plurality of conductors comprising a central conductor surrounded by at least a first peripheral layer of conductors comprising an even number of conductors symmetrically arranged around the central conductor.
The disclosure can be applied notably, but not exclusively, when the multi-conductor cable is a hepta-cable.
The disclosure can be applied notably, but not exclusively, for the transmission of signals between a first unit situated at a predetermined depth of a well and a second unit situated at the surface of the well.
In a particular case, the first unit and the second unit are respectively a telemetry unit and a surface central unit which are part of a seismic monitoring system.
Description of Related Art
It is sought more particularly here below in this document to describe problems existing in the field of seismic monitoring system. The disclosure of course is not limited to this particular field of application but is of interest for any solution for transmitting signals on a multi-conductor cable that has to cope with closely related or similar issues and problems.
Basically, and as shown in FIG. 1, a seismic monitoring system comprises some sensors 15a, 15b (also referred to as “seismic sensing tools”) disposed in a well 14 and along a tool bus (communication link) 16 and which transmit their data to a telemetry unit 11. The telemetry unit 11 later processes these data and sends them to a surface central unit 13 through a cable 12. The telemetry unit 11 is situated at a predetermined depth of the well (it might be disposed downhole, and associated with the seismic sensing tools, or at the top of the well, while the seismic sensing tools are far down).
The cable 12 linking the telemetry unit 11 and the surface central unit 13 is typically a hepta-cable as shown in FIG. 2, i.e. a multi-conductor cable comprising a plurality of conductors 1-7 surrounded by an armor 10. The plurality of conductors comprises a central conductor 7 surrounded by a peripheral layer of six conductors 1-6 symmetrically arranged around the central conductor 7.
A hepta-cable can provide for various signal propagation modes (also referred to as “transmission modes” or “configurations of transmission”), each of which transmits signals on a specific combination of the seven conductors 1-7 and armor 10.
When a single transmission is needed, the hepta-cable is usually used in a “common mode”, consisting in transmitting the signal over the central conductor 7 with a return on the armor 10 (and sometimes also on some or all of the peripheral conductors 1-6). This type of propagation mode provides a channel having a good quality.
FIG. 3-3 of EP2741427 discloses an example of such a common mode.
Another type of propagation mode in a hepta-cable is referred to as “differential mode”, consisting in delivering equal but opposite AC voltages and currents on a first conductor (or a set of first conductors) and a second conductor (or a set of second conductors). This type of propagation mode provides immunity to common mode noise.
FIG. 3-1 of EP2741427 discloses such a differential mode.
In order to increase the data rate, EP2741427 discloses a different use of the hepta-cable, with two simultaneous transmissions.
However, it appears that these multiple transmissions on the different conductors of the cable lead to the generation of cross-talk. The cross talk (also referred to as “channels' interference”) is an unwanted source of noise in data transmissions: the data transmitted via a conductor are polluted by the data transmitted simultaneously via another conductor of the same cable. Cross-talk may limit the available data rate and reliability.
Consequently, and as disclosed in detail in EP2741427, a hard data processing (or a use of a complex hardware) is needed because of cross-talk, when it is intended to increase the transmission rate.